# Questions tagged [turing-machines]

Questions about Turing machines, a theoretical model of mechanical computation capable of simulating any computer program.

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### How can I convert the Turing machine the recognizes language $L$ into an unrestricted grammar?

According to this Wikipedia article, unrestricted grammars are equivalent to Turing machines. The article notes that I can convert any Turing machine into an unrestricted grammar, but it only shows ...
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### Reduction of A_LBA to E_LBA

I have a rather interesting one to ponder and would love if I could get an answer for it. We were discussing the topic of mapping reduction today in my Computing theory course and I was wondering why ...
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### Robustness of Turing Machines - 3 dimensional case

How can one show that a machine with a three dimensional memory arranged in an infinite grid can be simulated by a single-tape Turing machine? I'd imagine there's some sort of mapping possible from ...
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### Multitape Turing machines against single tape Turing machines

Introduction: I recently learned that a multi-tape Turing Machine $\text{TM}_k$ is no more "powerful" than a single tape Turing machine $\text{TM}$. The proof that $\text{TM}_k \equiv \text{TM}$ is ...
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### Determining the classification of languages

$L_0 = \{ \langle M, w, 0 \rangle \mid \text{$M$halts on$w$}\}$ $L_1 = \{ \langle M, w, 1 \rangle \mid \text{$M$does not halt on$w$}\}$ $L = L_0 \cup L_1$ I need to determine where in ...
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### References on comparison between quantum computers and Turing machines

I was told that quantum computers are not computationally more powerful than Turing machines. Could someone kindly help in giving some literature references explaining that fact?
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### Visualizing a Non Deterministic Decider

I know that we can visualize a Non deterministic TM as a TM which splits into multiple copies of itself whenever it sees a non deterministic path (Yes, I also know that this is just a visualization ...
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### If a Language is Non-Recognizable then what about its complement?

Is the complement of a Non-Recognizable language Recognizable Non-Recognizable May be Recognizable, May be Non-recognizable. I.e cant comment. A mathematical proof would be of great help since im ...
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### determine if a machine prints a certain string in less time than it takes to run the machine itself?

Does there exist a procedure that determines if a polytime machine prints a certain string, and does so in less time than the machine itself takes to run? Define a machine $a$ that analyzes another ...
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### Are all context-sensitive languages decidable?

I was going through the Wikipedia definition of context-sensitive language and I found this: Each category of languages is a proper subset of the category directly above it. Any automaton and any ...
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### Why does it take $O(f(n)^2)$ to simulate a 3-tape $O(f(n))$-time TM on a 1-tape TM?

This looks like a fundamental result but I can't find a resource online that gives an intuitive interpretation of this complexity. Any basic explanation is appreciated.
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### Time complexity of an enumeration of SUBSET SUM instances

An instance of the SUBSET SUM problem (given $y$ and $A = \{x_1,...,x_n\}$ is there a non-empty subset of $A$ whose sum is $y$) can be represented on a one-tape Turing Machine with a list of comma ...
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### How to calculate the number of states in designing a Turing machine?

I would like to ask how to determine the number of states when designing a Turing machine from the description for a language? For example: $\qquad \displaystyle L = \{wcw \mid w \in \{0,1\}^*\}.$ I ...
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### What approaches are most useful when proving uncomputability of a given function?

I'd like to understand what approaches should one adopt when deciding/proving that a given function F is uncomputable, by any Turing Machine (TM). The ones I've tried so far are as follows: Reduction,...
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### What is the difference between halting, accepting, and deciding in the context of Turing machines?

Does accepting mean that the TM will read and recognize a char from the cell it's currently reading from? And is it the case that a TM halts iff the input is decidable?
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### Infinite alphabet Turing Machine

Is a Turing Machine that is allowed to read and write symbols from an infinite alphabet more powerful than a regular TM (that is the only difference, the machine still has a finite number of states)? ...
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### How does a single-track Turing machine simulate a multi-track Turing machine?

It's easy to see how a multi-track Turing machine can simulate a single-track Turing machine; it does so by ignoring all but the first track. But how does it work the other way? I need a specification ...
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### On-line simulation of a two-head tape Turing machine using single-head tape(s)

I have a question and I haven't been able to figure out the answer yet. I need to do the on-line simulation of a two-head tape Turing machine using single-head tape(s). I've found some online articles ...
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### Using a step-counting function in a Turing Machine construction

I have an question relating to the elementary foundations of Turing Machine theory. I would like to have a clarification of the status of a function $\phi$ (a mapping between TM indexes) I shall ...
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### Human computing power: Can humans decide the halting problem on Turing Machines?

We know the halting problem (on Turing Machines) is undecidable for Turing Machines. Is there some research into how well the human mind can deal with this problem, possibly aided by Turing Machines ...
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### Can the encodings set of a non-trivial class of languages which contains the empty set be recursively enumerable?

Let $C$ be a non-trivial set of recursively enumerable languages ($\emptyset \subsetneq C \subsetneq \mathrm{RE}$) and let $L$ be the set of encodings of Turing machines that recognize some language ...
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### Turing Recognisable => enumerable

I get the proof of going from an enumerator to a Turing Machine (keep running enumerator and see if it matches input) but I don't see how the other way works. According to my notes and the book (...
The document I am reading is here: Turing Machines Before getting into the question, here is the notation used on the picture: Here $\Delta$ denotes the blank and R, L and S denote move the head ...